ECU UPGRADE AND MANIFOLD UPGRADE

Ive put a 1.8 gti engine and gearbox into my si to convert it into a gti, but a rs 2000 lookalike. Ive put 2litre cams and throttle body on it but should i bother putting a 2litre manifold on? also, are the ecu's plug in? should i also change the MAF (MASS air flow ) sensor to. Help appreciated, thanks martin, any other tips to increase the speed and bhp would be good to.